Low molecular weight, acid stable proteinase inhibitors are present in the secretions nd tissues of the male reproductive tract. The majority of thes inhibitors are effective against the acrosomal enzyme, acrosin. This enzyme has been implicated in a number of processes directly related to fertilization. Thus the ability to control acrosin activity has significant potential for fertility regulation. The natural inhibitors found in the male reproductive tract offer one approach for the regulation of acrosin activity. We have previously demonstrated two (Epil and Epi2) low molecular weight inhibitors in murine epididymal and one in seminal vesicle homogenates. The lower molecular weight epididymal inhibitor (Epi2) differs significantly from that found in the seminal vesicles, suggesting a unique function for each of these inhibitors. We plan, using mice, to isolate by column chromatography Epi1 and compare its characteristics to those already determined for Epi2 and the seminal vesicle inhibitor. Furthermore, with anitbodies to each of the inhibitors we propose to determine 1) the specific sites of inhibitor production, 2) whether any or all of the inhibitors are secretory products 3) if and where they bind to the sperm and 4) where and when these inhibitors are removed from the sperm. Both the indirect immunofluorescent technique at the light microscopic level and the PAP method at the light and ultrastrcutral levels are applicable for these determinations. These studies should provide valuable, basic information concerning the characteristics and functions of these inhibitors and provide preliminary information for devising mechanisms for acrosin regulation.